A number of weeks ago I had my radiator hose blow and had to replace it. Early this week one day my wife picked up some things from town and parked the car in our driveway. She mentioned that it was odd that the radiator fan would come on after the car was shutoff (which it virtually NEVER did in the past). Immediately I did some troubleshooting. Eventually I found a pattern. When the key was in the ignition and the dash lights were on, one of the radiator fans would shut off. As soon as I turned the ignition off so that the dashboard lights were out, the radiator fan would come on. It is normal for this fan to come on if the engine is hot. In my case the temperature gauge on the dashboard showed that temperature was at half way (which is the highest it ever goes, unless something is wrong like a burst rad hose). After trying different things I could repeat like clockwork, turn off the ignition so the dash lights are off, and the fan comes on… and stays on for more than 15 minutes. I knew something was wrong.
After googling around for my problem I found MANY posts talking about things to try. Here are the steps to try to figure out what is wrong:
1. Under the hood, near the windshield on the passenger side you should see a box of relays, etc. On the cover it will tell you which relay is for the radiator fan. Notice the relay for power steering is the same type as the radiator fan. Try swapping the two (the fan and the power steering) to see if you have a bad relay. Try playing with the ignition and see if the fan goes off. In my case, the relay was fine and was not my problem. You can take out the fan relay in a pinch so as to not drain your battery until you can get the problem fixed. This puts your engine at a slight risk of over-heating because that fan will never turn on. In cold weather it isn’t much of a problem.
2. Check the Thermo Switch. Where is it? Follow the hose coming out of the top of the radiator towards the engine. At the end of the hose you will see a green plug. The thing that plug is inserted into is the Thermo Switch. This tells the fan when to turn on and off depending on rad fluid temperature. Be CAREFUL to get the right part. I read in many postings about the “cooling temperature sensor) which ended up wasting days due to getting the wrong part numerous times from the auto parts guys. Not sure if that is the same as the “thermo switch” but the name on my invoice was 37773-PT3-A01 Switch, Thermo. The price direct from Honda in Canadian dollars was $62.04. Unclip the green plug using a flathead screw-driver (come in from the end connected to the engine or you may break the green plug). Once you get it off, you will require a 24mm deep socket to get the old thermo switch off. Turn counter clockwise to unthread the old one. Rad fluid will leak out a bit (that is ok) once you get it out. Now put the new one in (thread clockwise) and hookup the green plug and voila, no more infinite fan syndrome! This is the most common problem for fans that either stay on or do not come on, a faulty thermo switch.
Here is a picture with the box of relays circled at the top left and the thermo switch circled in the center:
Hope this helps someone.